The influence of the flow separation bubble and transition location on the profile drag of three 4-digit NACA aerofoil profiles

2021 ◽  
pp. 0309524X2110550
Author(s):  
Moutaz Elgammi ◽  
Tonio Sant ◽  
Atiyah Abdulmajid Ateeah
Keyword(s):  
Wind Tunnel ◽  
Flow Separation ◽  
Separation Bubble ◽  
Reynolds Numbers ◽  
Force Balance ◽  
Navier Stokes ◽  
Separation Mechanism ◽  
Numerical Work ◽  
Low Reynolds Numbers ◽  
Low Reynolds

Modeling of the flow over aerofoil profiles at low Reynolds numbers is difficult due to the complex physics associated with the laminar flow separation mechanism. Two major problems arise in the estimation of profile drag: (1) the drag force at low Reynolds numbers is extremely small to be measured in a wind tunnel by force balance techniques, (2) the profile drag is usually calculated by pressure integration, hence the skin friction component of drag is excluded. In the present work, three different 4-digit NACA aerofoils are investigated. Measurements are conducted in an open-ended subsonic wind tunnel, while numerical work is performed by time Reynolds-averaged Navier Stokes (RANS) coupled with the laminar-kinetic-energy ( K-kl-w) turbulence model. The influence of the flow separation bubbles and transition locations on the profile drag is discussed and addressed. This paper gives important insights into importance of measurements at low Reynolds numbers for better aerodynamic loads predictions.

Effects of Flow Separation on a Highly Loaded, Low-Pressure Gas Turbine Blade at Low Reynolds Numbers

2015 ◽  
Author(s):  
Tyler M. Pharris ◽  
Olivia E. Hirst ◽  
Kenneth W. Van Treuren
Keyword(s):  
Wind Tunnel ◽  
Gas Turbine ◽  
Flow Separation ◽  
Separation Bubble ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Low Flow ◽  
Turbine Wheel ◽  
Low Pressure Turbine ◽  
Low Reynolds

Current gas turbine engines experience a loss in performance due to the low Reynolds number flow in the low-pressure turbine. This low flow speed can result in separation of the air from the blade surface, reducing the efficiency of the engine. The Baylor University Cascade wind tunnel (BUC) is being used to study this flow separation. A cascade wind tunnel contains a row of turbine vanes that simulates a turbine wheel. The BUC is capable of simulating the environment seen by the low-pressure turbine at high altitudes by producing Reynolds numbers varying from 25,000 to 400,000. The L1A blade profile is currently being tested. Coefficient of pressure (Cp) plots show a less than 1% difference between surface pressure locations when comparing the most inboard and outboard test blades. This agreement demonstrates the flow uniformity in the tunnel. Cp plots also compared favorably to the literature, validating the BUC operation and providing insight into how Reynolds numbers and free stream turbulence intensity (FSTI) affect flow separation. The literature and this study showed the size and reattachment of the separation bubble was highly dependent on the FSTI for lower Reynolds numbers (25,000 to 200,000). This comparison also showed that the size of the separation bubble and the location was not heavily impacted by FSTI for Reynolds numbers above 200,000. Tests in the future will be conducted to determine the actual FSTI of the BUC. Once completely validated, future studies with the BUC may include use of particle image velocimetry (PIV) to visualize the flow, a gold foil steady state technique using liquid crystals to measure heat transfer, and a series of deposition tests using surface roughness (sandpaper or textured sprays) to measure performance loss under these conditions. The ultimate goal of this research is to improve blade design in the low pressure turbine for all commercial and military aircraft.

Essential Ingredients for the Computation of Steady and Unsteady Blade Boundary Layers

1991 ◽  
Vol 113 (4) ◽  
pp. 608-616 ◽  
Author(s):  
H. M. Jang ◽  
J. A. Ekaterinaris ◽  
M. F. Platzer ◽  
T. Cebeci
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Stokes Equations ◽  
Inviscid Flow ◽  
Reynolds Numbers ◽  
Navier Stokes ◽  
Transitional Region ◽  
Low Reynolds Numbers ◽  
Flow Equations ◽  
Low Reynolds

Two methods are described for calculating pressure distributions and boundary layers on blades subjected to low Reynolds numbers and ramp-type motion. The first is based on an interactive scheme in which the inviscid flow is computed by a panel method and the boundary layer flow by an inverse method that makes use of the Hilbert integral to couple the solutions of the inviscid and viscous flow equations. The second method is based on the solution of the compressible Navier–Stokes equations with an embedded grid technique that permits accurate calculation of boundary layer flows. Studies for the Eppler-387 and NACA-0012 airfoils indicate that both methods can be used to calculate the behavior of unsteady blade boundary layers at low Reynolds numbers provided that the location of transition is computed with the en method and the transitional region is modeled properly.

Suppression of vortex shedding behind a circular cylinder by another control cylinder at low Reynolds numbers

2007 ◽  
Vol 573 ◽  
pp. 171-190 ◽  
Author(s):  
A. DIPANKAR ◽  
T. K. SENGUPTA ◽  
S. B. TALLA
Keyword(s):  
Vortex Shedding ◽  
Physical Mechanism ◽  
Grid Method ◽  
Reynolds Numbers ◽  
Accurate Solution ◽  
Navier Stokes ◽  
Navier Stokes Equation ◽  
Low Reynolds Numbers ◽  
Low Reynolds ◽  
Control Cylinder

Vortex shedding behind a cylinder can be controlled by placing another small cylinder behind it, at low Reynolds numbers. This has been demonstrated experimentally by Strykowski & Sreenivasan (J. Fluid Mech. vol. 218, 1990, p. 74). These authors also provided preliminary numerical results, modelling the control cylinder by the innovative application of boundary conditions on some selective nodes. There are no other computational and theoretical studies that have explored the physical mechanism. In the present work, using an over-set grid method, we report and verify numerically the experimental results for flow past a pair of cylinders. Apart from providing an accurate solution of the Navier–Stokes equation, we also employ an energy-based receptivity analysis method to discuss some aspects of the physical mechanism behind vortex shedding and its control. These results are compared with the flow picture developed using a dynamical system approach based on the proper orthogonal decomposition (POD) technique.

scholarly journals Force and flowfield measurements to understand unsteady aerodynamics of cycloidal rotors in hover at ultra-low Reynolds numbers

2019 ◽  
Vol 11 ◽  
pp. 175682931983367
Author(s):  
Carolyn M Reed ◽  
David A Coleman ◽  
Moble Benedict
Keyword(s):  
Separation Bubble ◽  
Fluid Dynamic ◽  
Unsteady Aerodynamics ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Dynamic Stall ◽  
Static Case ◽  
Low Reynolds Numbers ◽  
Curvature Effects ◽  
Low Reynolds

This paper provides a fundamental understanding of the unsteady fluid-dynamic phenomena on a cycloidal rotor blade operating at ultra-low Reynolds numbers (Re ∼ 18,000) by utilizing a combination of instantaneous blade force and flowfield measurements. The dynamic blade force coefficients were almost double the static ones, indicating the role of dynamic stall. For the dynamic case, the blade lift monotonically increased up to ±45° pitch amplitude; however, for the static case, the flow separated from the leading edge after around 15° with a large laminar separation bubble. There was significant asymmetry in the lift and drag coefficients between the upper and lower halves of the trajectory due to the flow curvature effects (virtual camber). The particle image velocimetry measured flowfield showed the dynamic stall process during the upper half to be significantly different from the lower half because of the reversal of dynamic virtual camber. Even at such low Reynolds numbers, the pressure forces, as opposed to viscous forces, were found to be dominant on the cyclorotor blade. The power required for rotation (rather than pitching power) dominated the total blade power.

Actuated Transition in an LP Turbine Laminar Separation: An Experimental Approach

2011 ◽  
Author(s):  
Jenny Baumann ◽  
Ulrich Rist ◽  
Martin Rose ◽  
Tobias Ries ◽  
Stephan Staudacher
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Reynolds Numbers ◽  
Experimental Investigations ◽  
Stream Velocity ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Low Reynolds ◽  
Lp Turbine

The reduction of blade counts in the LP turbine is one possibility to cut down weight and therewith costs. At low Reynolds numbers the suction side laminar boundary layer of high lift LP turbine blades tends to separate and hence cause losses in turbine performance. To limit these losses, the control of laminar separation bubbles has been the subject of many studies in recent years. A project is underway at the University of Stuttgart that aims to suppress laminar separation at low Reynolds numbers (60,000) by means of actuated transition. In an experiment a separating flow is influenced by disturbances, small in amplitude and of a certain frequency, which are introduced upstream of the separation point. Small existing disturbances are therewith amplified, leading to earlier transition and a more stable boundary layer. The separation bubble thus gets smaller without need of a high air mass flow as for steady blowing or pulsed vortex generating jets. Frequency and amplitude are the parameters of actuation. The non-dimensional actuation frequency is varied from 0.2 to 0.5, whereas the normalized amplitude is altered between 5, 10 and 25% of the free stream velocity. Experimental investigations are made by means of PIV and hot wire measurements. Disturbed flow fields will be compared to an undisturbed one. The effectiveness of the presented boundary layer control will be compared to those of conventional ones. Phase-logged data will give an impression of the physical processes in the actuated flow.

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